This invention relates to a device for filtering polyenergetic X-ray beams to obtain emergent beams which have average photon energies lying within a relatively narrow spectral band. The new filter device is especially useful in X-ray apparatus that is adapted for performing digital subtraction angiography.
Digital subtraction angiography (DSA) is now used to image vessels which contain blood that entrains an X-ray contrast medium such as an iodinated compound. According to one DSA method, X-ray beams having low average photon energy and relatively higher average photon energy are projected through a patient alternatingly and in rapid succession over a pre-contrast interval during which the contrast medium has not reached the blood vessel of interest and continuing over the succeeding post-contrast interval during which the medium enters, maximizes opacity and leaves the vessel. In one method, for example, the digital pixel data that represents the image obtained with the first low energy exposure is treated as a mask image and all succeeding pre-contrast and post-contrast low energy images are subtracted from the low energy mask image and the result is stored. Likewise, the first high energy exposure yields mask image data and the data for all succeeding pre-contrast and post-contrast high energy images is subtracted from the high energy mask and the results are stored. This is commonly called temporal subtraction. The stored low and high energy images are then substracted from each other in what is called energy subtraction to yield the data for a difference image frame which should contain pixel data representative only of the blood vessel whose interior is outlined by the boundaries of the contrast medium. The image data are usually weighted before subtraction in such manner that bone and soft tissue will be subtracted out and the contrast medium-filled vessel representative data will remain. The procedure just outlined is commonly called hybrid digital subtraction angiography because there is a temporal subtraction step and an energy subtraction step involved.
Low and high average photon energy X-ray beams are obtained by applying low and high kilovoltages to the anode of an X-ray tube, respectively. As a result, the peak energies of the X-ray photons are substantially different from each other for the low and high kilovoltages but each of the beams will have a distribution of X-ray photon energies. Subtraction of unwanted parts of the images such as soft tissue and bone can be improved if suitable filter elements are put in the X-ray beam which filter out photons having energies below the desired energies of the high and low kilovoltage beams.
The X-ray images are usually acquired with an X-ray image intensifier that converts them to optical images. The optical images are viewed with a television (TV) camera. A preferred operating mode is to make a low energy exposure while the TV camera is blanked and then read out the camera target during the next frame time of 33 ms when the X-ray beam is turned off. This is followed immediately by making an exposure with the high energy X-ray beam while the TV camera is blanked and then reading out the camera target during the next frame time while the X-ray beam is turned off. Thus, it will be seen that for optimum results it is desirable to be capable of inserting one of the X-ray filters into the X-ray beam path before the exposure at one energy is started and to remove the same filter and insert another filter during a TV camera readout frame time of typically 33 ms so the filter will be stationary by the time the exposure at the other X-ray energy is started. Thus, the most rigorous requirements are to be able to insert a filter of one type in the X-ray beam path, stop it, hold it for the duration of an exposure at one energy, remove it and insert another filter during a camera readout frame time and stop the filter and hold it steady during the exposure at the other energy. Some techniques call for making on the order of 50 high and low energy exposure pairs within about a 25-second interval by way of example. In any case, it is desirable to be able to make the high energy and low energy exposures in a pair as close to each other as is possible to minimize the adverse effects of involuntary tissue movement which would result from the exposures being separated from each other by a substantial amount of time.
The most common practice for inserting different filters in an X-ray beam is to mount the filters on a disk or drum that is driven rotationally in one direction and to attempt to make the X-ray exposures in synchronism with arrival of the proper filter in the X-ray beam. These filter devices must have the filter element rotating about a large radius which means that the filter wheel must have a large radius. It will thus have high inertia prohibiting abrupt acceleration, pausing at a stop and then accelerating and pausing again to have another filter element in the beam before starting the next exposure. A proposed solution to this is to use filter elements that are formed as long arcs and to have them mounted on a filter wheel of substantial radius such that the arcuate filter elements will remain in the X-ray beam long enough to permit exposures at different energies for limited durations at least. A filter device of this kind would have to be unacceptably large and difficult to locate close to the X-ray tube focal spot.